I have never read it this way. It's in Henry's best interest to endear himself to Mrs. Allen, since she's Catherine's chaperone. If she brings up a topic in which he is well-versed, why wouldn't he jump on it? The only bit of insidiousness I can suspect him of, now that you mention it, is letting her say the price before giving his assessment of it. If she had turned to him and asked, "And how much do you think I paid for this muslin, sir?" it would have been really impressive for him to come up with 9 shillings a yard.

Now, the narrator tells us that Catherine feared, as she listend to their discourse [on muslin and shopping], that [Henry] indulged himself a little too much with the foibles of others.